docs: clarify local plugin install target and clone options

[shuofengzhang]

What changed

• Updated local testing instructions in CONTRIBUTING.md to use the explicit install target:
  • claude plugin install sentry-skills@sentry-skills
• Updated README.md local development commands to:
  • include both SSH and HTTPS clone options
  • use the same explicit install target (sentry-skills@sentry-skills) for consistency

Why

• The repo had mixed install command variants (sentry-skills vs sentry-skills@sentry-skills).
• Using one explicit command in both contributor docs reduces ambiguity and makes setup steps more copy/paste-friendly.
• Adding an HTTPS clone option helps contributors who do not have GitHub SSH configured.

Testing

• scripts/clone_and_test.sh getsentry/skills
• Tests: not run (no tests found)

[shuofengzhang]

CI is blocked by required workflow configuration, not by this PR diff: both review and warden fail at actions/create-github-app-token with Input required and not supplied: app-id. I also cannot rerun those jobs (workflow file may be broken). This needs maintainer-side required-workflow/app secret configuration.